Dzhokhar Tsarnaev was an all-star wrestler at Cambridge Rindge and Latin High School.

The younger one — the one their father described as “like an angel” — gathered around him a group of friends so loyal that more than one said they would testify for him, if it came to that.

The older one, who friends and family members said exerted a strong influence on his younger sibling — “He could manipulate him,” an uncle said — once told a photographer, “I don’t have a single American friend. I don’t understand them.”

A kaleidoscope of images, adjectives and anecdotes tumbled forth on Friday to describe Tamerlan Tsarnaev, 26, and Dzhokhar Tsarnaev, 19, the two brothers suspected of carrying out the bombings at the Boston Marathon that killed three people and gravely wounded scores more.

What no one who knew them could say was why the young men, immigrants of Chechen heritage, would set off bombs among innocent people. The Tsarnaevs came with their family to the United States almost a decade ago from Kyrgyzstan, after living briefly in the Dagestan region of Russia. Tamerlan, who was killed early Friday morning in a shootout with law enforcement officers, was 15 at the time. Dzhokhar, who was in custody Friday evening, was only 8.

In America, they took up lives familiar to every new immigrant, gradually adapting to a new culture, a new language, new schools and new friends.

Dzhokhar, a handsome teenager with a wry yearbook smile, was liked and respected by his classmates at Cambridge Rindge and Latin School, where celebrities like Ben Affleck and Matt Damon had walked the halls before him. A classmate remembered how elated he seemed on the night of the senior prom. Wearing a black tuxedo and a red bow tie, he was with a date among 40 students who met at a private home before the event to have their photos taken, recalled Sierra Schwartz, 20.

“He was happy to be there, and people were happy he was there,” Ms. Schwartz said. “He was accepted and very well liked.”

A talented wrestler, he was listed as a Greater Boston League Winter All-Star. “He was a smart kid,” said Peter Payack, 63, assistant wrestling coach at the school. In 2011, the year he graduated, was awarded a $2,500 scholarship by the City of Cambridge, an honor granted only 35 to 40 students a year.

For Tamerlan, life seemed more difficult.

A promising boxer, he fought in the Golden Gloves National Tournament in 2009, and he was noticed by a young photographer, Johannes Hirn, who took him as a subject for an essay assignment in a photojournalism class at Boston University. “There are no values anymore,” Tamerlan said in the essay, which was later published in Boston University’s magazine The Comment. “People can’t control themselves.”

Anzor Tsarnaev, the brothers’ father, who returned to Russia about a year ago, said in a telephone interview there that his older son was hoping to become an American citizen — Dzhokhar became a naturalized citizen in 2012, but Tamerlan still held a green card — but that a 2009 domestic violence complaint was standing in his way.

“Because of his girlfriend, he hit her lightly, he was locked up for half an hour,” Mr. Tsarnaev said. “There was jealousy there.” Tamerlan later married and had a small child. He was interviewed by the F.B.I. in 2011 when a foreign government asked the bureau to determine whether he had extremist ties, according to a senior law enforcement official.

Yet Dzhokhar admired and emulated his older brother.

Peter Tean, 21, a high school wrestling teammate, said that he thought Dzhokhar’s intense interest in rough-and-tumble sports came from a desire to be like his brother.

“He’s done these violent sports because his brother’s a boxer,” Mr. Tean said. “He really loves his brother, looks up to him.”